Abstract
Knee-extensors demonstrate greater fatigue resistance in females compared to males during single-limb and whole-body exercise. For single-limb exercise, the intensity-duration relationship is different between sexes, with females sustaining a greater relative intensity of exercise. This study established the power-duration relationship during cycling, then assessed fatigability during critical power-matched exercise within the heavy and severe intensity domains. When critical power and the curvature constant were expressed relative to maximal ramp test power, no sex difference was observed. No sex difference in time to task failure was observed in either trial. During heavy and severe intensity cycling, females experienced lesser muscle de-oxygenation. Following both trials, females experienced lesser reductions in knee-extensor contractile function, and following heavy intensity exercise, females experienced less reduction in voluntary activation. These data demonstrate that whilst the relative power-duration relationship is not different between males and females, the mechanisms of fatigability during critical power-matched exercise are mediated by sex. Due to morphological differences, females demonstrate greater fatigue resistance of locomotor muscle during single-limb and whole-body exercise modalities. Whilst females sustain a greater relative intensity of single-limb, isometric exercise than males, limited investigation has been performed during whole-body exercise. Accordingly, this study established the power-duration relationship during cycling in 18 trained participants (eight females). Subsequently, constant-load exercise was performed at critical power (CP)-matched intensities within the heavy and severe domains, with the mechanisms of fatigability assessed via non-invasive neurostimulation, near-infrared spectroscopy and pulmonary gas exchange during and following exercise. Relative CP (72±5 vs. 74±2% Pmax , P=0.210) and curvature constant (51±11 vs. 52±10JPmax-1 , P=0.733) of the power-duration relationship were similar between males and females. Subsequent heavy (P=0.758) and severe intensity (P=0.645) exercise time to task failures were not different between sexes. However, females experienced lesser reductions in contractile function at task failure (P≤0.020), and greater vastus lateralis oxygenation (P≤0.039) during both trials. Reductions in voluntary activation occurred following both trials (P<0.001), but were less in females following the heavy trial (P=0.036). Furthermore, during the heavy intensity trial only, corticospinal excitability was reduced at the cortical (P=0.020) and spinal (P=0.036) levels, but these reductions were not sex-dependent. Other than a lower respiratory exchange ratio in the heavy trial for females (P=0.039), no gas exchange variables differed between sexes (P≥0.052). Collectively, these data demonstrate that whilst the relative power-duration relationship is not different between males and females, the mechanisms of fatigability during CP-matched exercise above and below CP are mediated by sex.
Highlights
The exercise intensity–duration relationship is a phenomenon that permits mechanistic insight into the metabolic demands and physiological consequences of exercise within distinct intensity domains (Jones et al 2010; Poole et al 2016; Burnley & Jones, 2018)
When data were expressed in absolute units, males demonstrated greater values than females (P ࣘ 0.009), but when critical power (CP) and W were normalised to maximal power (Pmax), no differences between the sexes were observed (P ࣙ 0.210)
The present study explored the sex difference in fatigability during locomotor exercise by comparing the power–duration relationship, muscle oxygenation and neuromuscular responses to CP-matched exercise intensities
Summary
The exercise intensity–duration relationship is a phenomenon that permits mechanistic insight into the metabolic demands and physiological consequences of exercise within distinct intensity domains (Jones et al 2010; Poole et al 2016; Burnley & Jones, 2018). Recent evidence has shown that the power–duration relationship differs between males and females for intermittent, isometric knee-extensor exercise (Ansdell et al 2019a). The mechanism for this is probably a result of morphological differences within the exercising musculature in comparison to males. VL type I fibre proportion (Vanhatalo et al 2016) and capillarisation (Mitchell et al 2018a) are positively correlated with aerobic exercise performance indices such as CP during cycling in males and mixed-sex samples, providing a potential explanation for why females are able to sustain a greater relative exercise intensity than males during an isometric exercise paradigm (Ansdell et al 2019a)
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